1. Field of the Invention
The present invention relates to a multi-beam optical head for obtaining reproduced information signals and various kinds of servo signals by utilizing light reflected by a recording medium, and, in particular, to a multi-beam optical head having a mechanism for adjusting the position of a light source relative to that of a head optical system and a method for adjusting the position of the light source relative to that of the head optical system.
2. Description of the Related Art
A multi-beam optical head is an optical head which leads a plurality of beams emitted from a monolithic light source having a plurality of emitting points, e.g. a semiconductor laser, to an identical optical system and converges the beams on a recording surface of a recording medium, e.g. an optical disc, so as to record and reproduce the information of the respective beams. When such a multi-beam optical head is used, a focus error signal is detected by using any one of the plurality of beams passing through an identical objective lens to conduct a focus servo so that the selected beam may be converged on the recording medium more precisely than any other beam. In conducting the focus servo, in order to situate the focal points of all the beams on the recording medium, i.e. on the same plane, the deviation of the focal point of the beam to be subjected to the focus servo from those of the other beams should be sufficiently smaller than the focal depth of the beam.
The distance from an optical system to the focal point of a beam depends on the distance from the emitting point of a light source to the optical system. Since the positions of the emitting points of a multi-beam light source deviate from each other because of the errors occurring in the production process, the distances from the emitting points to the optical system are different in their respective beams. Accordingly, the distances from the optical system to the focal points of the respective beams are also different. The amount of the deviation is determined depending on the amount of the error of the position of the emitting point and the image magnification of the optical system. For example, if the deviation of given two emitting points is assumed to be 4 .mu.m and the image magnification of an optical system is 1/4 as actual values, the deviation of the positions of the focal points is approximately 1 .mu.m. This error of the relative positions of the focal points is significantly large compared with the standard focal depth of about 1 to 2 .mu.m of a currently available optical head.
As a conventional technique for solving the above-mentioned problem, there is proposed a technique disclosed in Japanese Laid-Open Patent Publication No. 3-40230. The multi-beam optical head according to this technique includes a signal detecting optical system for detecting focus error signals based on a first beam for an ordinary focus servo and an additional second beam; and a driving means for controlling the light source by rotating it so as to eliminate the deviation of the focal point of the second beam based on the detected focus error signal. And the optical head conducts the correction of the focal point of the second beam in addition to the focus servo of the first beam in order to restrict the deviation of the focal points of all the beams to the acceptable range.
When such a technique is employed, however, the configuration of the head becomes disadvantageously complicated because an additional optical system for detecting signals or an additional driving mechanism is required as well as an ordinary focus servo mechanism. The major cause of the deviation of the focal points is the deviation of the positions of the emitting points caused by the errors of the position of the light source occurring during the production process. However, the positions of the emitting points never deviate during the operations of the optical head. Accordingly, it is not necessarily indispensable to conduct such corrections of the focal points at a real time during the operations of the optical head and it is not preferable to make the configuration complicated just for this purpose. Furthermore, in the controls to correct the focal point, a signal control as well as the ordinary focus control must be conducted, so it is difficult to design a servo system in which the entire system may operate smoothly.